1. Field of the Invention
This invention relates to the purification of a molten metal by filtration. More particularly, this invention relates to a two-stage filtration system, including method and apparatus for the purification of molten metal using coarse and fine rigid filter media.
2. Description of the Related Art
The removal of impurities from a molten metal such as aluminum has been previously accomplished by passing a chlorine-containing gas through the molten aluminum, by passing the molten aluminum through filtration means, or using a combination of same.
For example, Stroup et al U.S. Pat. No. 2,840,463 teaches the purification of aluminum by dripping the aluminum through a perforated plate into a gas-tight vessel filled with aluminum chloride gas which is used to degas and purify the aluminum metal as it drips into the vessel.
Molten metal has also been purified by passing it through a bed of particles or granules which provides filtration without caking or clogging of the filter. For example, Brondyke et al U.S. Pat. No. 2,863,558 describes the purification of molten aluminum to remove finely divided non-metallic particles or inclusions by passing the aluminum metal through a bed of refractory particles.
The use of more than one size of refractory particles in the filtration bed is shown in Lee et al U.S. Pat. No. 3,025,155, Hess et al U.S. Pat. No. 3,039,864 and Blayden U.S. Pat. Nos. 3,737,303 and 3,737,305. In these patents, a gas is bubbled countercurrently through the bed which may comprise a chlorine-containing gas capable of reacting with the impurities in the molten aluminum, or a non-reactive gas, which may be used to remove occluded gases from the molten aluminum as well as to provide a backflushing of the particle bed.
Blayden et al U.S. Pat. No. 3,737,304 describes an apparatus and method for purifying aluminum by passing it through two such beds of refractory granules while both chlorine and non-reactive gases are passed countercurrently. The second filter bed differs from the first bed in that only coarse refractory granules are present in the first bed, while a bed of smaller sized refractory granules is located over a bed of coarse granules in the second bed.
Yu U.S. Pat. No. 4,384,888 shows a method of purifying molten aluminum by passing it through a medium of submerged contacting surfaces, such as found on Raschig rings or Interloc saddles, while passing a gas flux through the metal. The patentee prefers such contact surface media over the use of particles because of the larger void fraction obtainable by such usage. Periodically, gas is passed through the bed at two or three times the normal rate to purge or dislodge materials trapped in the bed to cause such materials to rise and collect as a floating layer. A single layer of 3/4 inch to 1 inch refractory balls on the molten metal surface are said to restrain the rings and saddles from leaving the bed during the high rate purging gas flow.
Non-particulate filter means have also been used in the filtration of molten aluminum. Eckert U.S. Pat. No. 4,769,158 describes a method and system for filtering a molten metal using a movable and flexible filter cloth which separates a first chamber from a second chamber in a filtering apparatus. The filter cloth may be gradually unrolled from a roll outside the first chamber to provide new filtration surfaces as particles collect on (and eventually clog) the filter cloth. Secondary filtration comprising a static filter is provided in the second chamber to trap any inclusions which dislodge from the filter cloth.
It would, however, be desirable to have rigid filter means capable of removing both coarse and fine impurities from a molten metal while inhibiting the usual clogging of a rigid filter and wherein the rigid filter means may be easily removed for replacement.